Hmm, I don't really agree with the interpretation that Dracula revived in the underworld. This isn't directly supported by the story itself. If it were true, you would think this would have been stated explicitly in the dialogue. (Which in Castlevania's case is always heavy on exposition, so the stories tend to spell things out directly for the audience instead of leaving it open to interpretation).
Shanoa: "There are magic energies coursing through this room... If that statue is Cerberus, then this must be the gateway into Hell. I just need to figure out how to unseal it..."
It can't get more explicit.
I digress.. You're drifting away from your initial question.
What you believe re: the events of OOE doesn't alter Barlowe's motives. His aim was to resurrect Dracula, simple enough, which initially Shanoa was meant to do for him by using Dominus on the seal. The simplest reason being that (1) he didn't initially believe there was another way to break the seal aside from using Dominus and (2) Given the choice, he wouldn't use his own life. The Dominus method was the "correct method", while the method which was used in the end was not initially foreseen by Barlowe. But in the end he'd rather see Dracula resurrected because the he couldn't defeat Shanoa, so he had no choice but to use his own life.
I also feel the "Why didn't Barlowe just self destruct prior to the Shanoa fight?" is not a good one. He obviously didn't previously have the means to break the seal (aside from Dominus) which is why the entire Shanoa>Albus>Dominus escapade started with the game's prologue. By the end of the Shanoa fight, Barlowe is spent and seems weak, when -let's call it- "Chaos" enters his body, he remarks about being re-energised/ power restored and then proceeds to give his own life, he had no other options.
If you focus on the underworld/ hell component then you've lost sight of your initial question. The only thing this component being a factor could explain is whether Dracula's resurrection is incomplete. If one discounts this component then that uncertainty is no longer a factor.
FYI, you, myself and plottwist discussed this 2 years back
http://castlevaniadungeon.net/forums/index.php?topic=8243.15Anyway, how about this one? Mathias' goal in LoI was to obtain a vampire's soul so he could become one himself and thus obtain eternal life, correct? Why couldn't he just have absorbed the soul of a minor vampire who is easy to kill? For example, why not take Joachim's soul? Mathias was in the same room when Leon defeated Joachim, so why didn't he do it?
To do with Mathias, are you referring to this:
https://youtu.be/ue_vmHmcT9g at 10:20 timestamp? (If this is even Mathias inside the room, I'm not sure if it is why he appears translucent..)
However, there could be several reasons why (assuming at this point that Mathias is already in possession of the stone and it is him). Initially Mathias asked Leon to join him to curse and defy God, Mathias is tactician he knows to expect all outcomes and plan for the worst. Joachim was not enough power for Mathias.
Perhaps Mathias initially believed Leon may attempt to kill him upon finding out he was responsible for what happened to Sara. This is plausible imo, Mathias he probably wasn't aware whether the Ebony Stone would shatter, presuming Leon would defeat Walter. He then fled purely because of daybreak, leaving Leon to Death. If the Ebony stone wasn't destroyed them I'm sure Mathias would've taken no issue using Walter's power to fight Leon (taking the ebony stone for himself also).
The Joachim fight could've been a testing ground for Mathias to observe Leon. Why not just opt for the higher power in any case? Mathias doesn't seem like he'd do something half-assed and Walter is clearly the superior power.